Some can, some can't - it really depends on what kinds of "effects" and/or special processing you want to have in your photos. I mean, for basic tone & color etc., Lr5 has all you need. But if you want to run LucisArt, or take advantage of content-aware fill, or do things with layers..., you need PS...

If you consider yourself primarily a photographer, sticking to Lightroom only will probably improve your work. The problem with Photoshop is that it becomes all too tempting to think "I'll fix that later" instead of getting the shot right to begin with.

If you take a look over in the PS forum you'll see lots of threads like "how do I remove these reflections", "how do I clean up this shot", or the all-out "how do I fix this picture". In all cases, almost without exception, the answer is take ten minutes extra and do it right the first time...Photoshop encourages laziness and bad habits, by its very nature.